A coaxial cable most often comprises an inner metallic signal conductor, a dielectric system surrounding the inner conductor, and an outer electrically conductive shield member surrounding the dielectric system. A suitable electrically conductive metal such as copper or a copper alloy, aluminum, or an iron alloy, such as steel, is used as the center signal conductor and in the form of a tube, a braided mesh or jacket, or as a layer of dielectric tape is used to surround the exterior of the cable as a shield against extraneous electric signals or noise which might interfere with any signals being carried by the center conductor.
The best available dielectric, theoretically, which could be used would be air, which has a dielectric constant of 1.0. Since it is almost impossible to construct a cable having only an air dielectric, practical cables of use in commerce must utilize materials and/or constructions allowing an approach as close as is possible to a dielectric constant of 1.0, while at the same time retaining adequate strength, flexibility, waterproofness, other desirable electrical properties in addition to minimum dielectric constant, and other properties of value in the art of coaxial electric cables.
The approach of foaming a dielectric, such as polyethylene about the center conductor, then surrounding the foam by unfoamed dielectric has been taken by Gerland, et al, in U.S. Pat. No. 3,516,859 and Griemsmann in U.S. Pat. No. 3,040,278. A spiral rib made from dielectric material was wound about a conductive center core to space the core from a dielectric or conductive metal tube surrounding and concentric with the conductive core by Saito, et al in U.S. Pat. No. 4,346,253, and Hildebrand, et al, in U.S. Pat. No. 3,286,015, to provide as much air dielectric as possible surrounding the conductive signal center core. Dielectric strands have been wound spirally about conductive center cores for the same purpose by Lehne, et al, in U.S. Pat. No. 2,197,616, Hawkins, in U.S. Pat. No. 4,332,976, Bankert, Jr., et al, in U.S. Pat. No. 3,750,050, in a waveguide structure, and by Herrmann, Jr., et al, in U.S. Pat. No. 4,018,977, in high voltage power cable. Disc type spacers have also been tried, being strung at intervals down a conductive center wire leaving air between them. This and some of the other constructions, however, lack mechanical strength, particularly when a cable is bent, and use of more material to add strength also increases weight and bulk, which is detrimental for many uses, such as space devices or computer equipment.